Chiral Organic Structure-Directing Agents

Abstract

Chirality is crucial for life. The preparation of enantiopure chiral compounds is highly desirable in the chemical industry, especially in the pharmaceutical sector. In this context, the design of chiral solids able to discriminate between enantiomers of chiral compounds, either during adsorption or asymmetric catalytic processes, is one of the greatest challenges nowadays in chemical research. Zeolite-type materials represent ideal candidates to achieve enantioselective chiral solids since they could combine their high stability, surface area and shape-selectivity with a potential enantioselectivity that could be enhanced by the confinement effect. Despite the occurrence of chiral zeolite frameworks and the strong interest in preparing these chiral solids, very little success has been met in preparing these in homochiral form. The main strategy to induce chirality in zeolite materials has been the use of chiral structure-directing agents, in an attempt to transfer their chiral feature into the nascent zeolite structure. However, although many chiral organic species have directed the crystallization of zeolite frameworks, some of them even being chiral, there is only one unique very recent example of success in transferring the chirality from the organic structure-directing agent into an enantio-enriched chiral zeolite material. Chiral coordination compounds have been very successful in transferring their chirality onto inorganic frameworks through the development of extensive H-bond host-guest interactions, but these chiral materials usually collapse upon removal of the guest species. In this chapter we report the different types of chiral molecules, both organic and organometallic compounds, used so far as structure-directing agents in an attempt to promote the crystallization of homochiral zeolites; we analyze in detail the possible reasons for the general failure in transferring their chirality, and we propose approaches to prepare known chiral zeolite frameworks in homochiral form. Furthermore, we also review a different approach we have followed in our group in order to induce chirality in zeolite materials, consisting in the development of chiral spatial distributions of dopants embedded in otherwise achiral zeolite frameworks.

Synthesis–Structure–Activity Relations in Fe-CHA for C–H Activation: Control of Al Distribution by Interzeolite Conversion

“…Si-islanding is a well-documented phenomenon in silicoaluminophoshates. [58][59] Even more relevant here is the intra-framework T-atom migration proposed by the Hong group upon finding heteroatom reshuffling in gallosilicates at prolonged synthesis times. 60 The DCC trends at various synthesis temperatures seem to confirm that Al redistribution mechanisms are at play to dynamically change the DCC values.…”

GTM-3, an Extra-Large Pore Enantioselective Chiral Zeolitic Catalyst

“…However, for such a transfer of chirality to occur, a true template effect, in the sense of a close geometrical relationship established between SDAs and zeolite frameworks, must occur during crystallization. This is not often the case, and although many chiral organic cations have been used over the years, , only very recently this strategy has been, for the very first time, proved successful for producing an enantio-enriched chiral zeolite. ,, Davis and co-workers prepared a rationally designed organic cation where the asymmetric nature of the SDA was transferred to the STW chiral framework, resulting in enantio-enriched materials. Interestingly, the authors found enantioselective properties of their material both in adsorption and catalytic processes.…”

A combination of proton spin diffusion NMR and molecular simulations to probe supramolecular assemblies of organic molecules in nanoporous materials

“…Therefore, the study at molecular level of the structure-directing effect of chiral organic molecules during the synthesis of zeolite microporous materials is fundamental in order to realize a transfer of chirality from organic species to inorganic zeolite frameworks. 7 This has been recently accomplished for the STW chiral framework by using computationally-designed chiral SDAs that follow the helicoidal pattern of the micropores of the STW network. 8,9…”